Positive displacement compressors in air conditioning and refrigeration applications are normally operated over a range of capacities and thus require some means for modifying their operation if efficient operation is to be maintained. It is desirable to be able to unload a compressor to various percentages of capacity in fixed increments, or continuously, over an entire range. Simultaneously, it is desirable to efficiently maintain the discharge pressure to suction pressure ratio, or Vi, for meeting system requirements. To meet these various requirements, a number of individual controls are used. In the case of helical screw compressors, for example, capacity control is conventionally achieved by the use of a slide valve. The slide valve is located in and slides axially in the cusp of the housing formed between the intersecting bores of the two rotors. The slide valve thus defines a portion of each bore and thereby compromises the integrity of the housing as well as making for a complicated device. The slide valve is reciprocatably positionable with respect to the axes of the rotors and can thus effectively change the start of compression by changing the closing point of the suction volume and thereby controlling the amount of gas trapped and compressed. Axial type slide valves can also be placed in various positions around the rotor bores defining a portion of one bore only. Additionally, axial slot valves displaced from the rotor bores are used.
An axial slide valve is provided with an axially extending fluid chamber at each end of the slide valve such that the slide valve is acted on by fluid pressure during compressor operation and may always be biased towards an open or unloaded position by a spring. Typically, the force of the spring acts in conjunction with suction pressure in one of the chambers in opposition to the discharge pressure or pressure supplied by a lubricating pump, or the like, to the opposing chamber which is sealed by a fixed piston. At start up, with the fluid pressures balanced, the spring bias will act on the slide valve to position it in a position corresponding to the lowest compressor capacity which makes starting the compressor easier. As the discharge pressure or the lubricating pump pressure builds up in the opposing chamber and acts on the valve causing it to move against suction pressure and the spring bias, the spring is thereby compressed and the valve increases the volume available for compressing gas. The force differential acting on the valve will determine the position of the valve and thereby the magnitude of the trapped volumes and thus the pumping capacity of the compressor. Because the fluid chambers are located within the slide valve and provide the location for the spring and fixed piston, the control structure is very compact.
It is an object of this invention to provide a compact control mechanism for axial slide valves.
It is an additional object of this invention to provide Vi control for partial load operation of an air conditioning compressor.
It is another object of this invention to provide automatic unloading for a compressor at start up.
It is a further object of this invention to increase the minimum required rotational speed for variable speed screw compressors.
It is an additional object of this invention to automatically achieve optimum Vi to match up the pressure differential for partial loading. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, an axial slide valve is provided with an axially extending fluid chamber at each end with one chamber receiving a spring and being acted on by suction pressure and the other chamber coacting with a fixed piston and being acted upon by discharge pressure, or the like, whereby the slide valve is positioned so as to balance the spring and fluid pressures and thereby regulate the compressor capacity.